In recent years, a touch panel-equipped display device in which a display device is combined with a touch panel is put in practical use, and is used for a wide variety of applications. Touch panels have various types, but among them, capacitive type touch panels are widely used.
In a touch panel-equipped display device, in order to detect an object to be detected reliably, it is effective to remove noise from a signal detected by a detection electrode. By removing noise, a change in output signals between when an object to be detected is detected and when an object to be detected is not detected can be made clearer. This makes it possible to discern a detection state from a non-detection state more reliably. In other words, by increasing S/N ratio by removing noise components from a signal detected by a detection electrode, the sensitivity of the touch panel can be improved.
A display device is not always driven or scanned for performing image display. In one frame of an image, there is a period called a vertical blanking period, in addition to a display period. In the case of LCDs, for example, during the vertical blanking period, pixels merely hold the states written by thin film transistors (TFTs), and therefore, an environment with little noise for a touch panel is realized. Thus, by conducting a sensing operation of a touch panel during this vertical blanking period, it is expected that noise detected by a detection electrode of the touch panel is reduced.
On the other hand, the vertical blanking period for a display device is a period during which a certain display state is maintained, but if all operations are stopped, it is not possible to maintain the display state in a desired manner (Patent Document 1). In the case of LCDs, for example, each pixel is provided with a storage capacitance to maintain a display state written into by a TFT, but the accumulated charges are discharged over time. This causes brightness of each pixel to change over time, resulting in flickering, and display quality is thereby lowered.
Patent Document 1 discloses a technology for eliminating a difference in brightness among pixels by inverting an output voltage of a display signal circuit (13) (source bus line below) for every horizontal scanning period during the vertical blanking period.